Borehole survey systems used for geological surveying, mining and the drilling of oil and gas wells generally map or plot the path of a borehole by determining borehole azimuth (directional heading relative to a reference coordinate such as north) and borehole inclination (relative to vertical) at various points along the borehole. For example, in one early type of prior art system, a tool or probe that contains one or more magnetic or gyroscopic compasses for indicating azimuth and one or more pendulums or accelerometers for indicating inclination is suspended by a cable and raised and lowered through the borehole. In such a system, the position of the probe along the borehole is determined by the length of cable that extends between the entrance of the borehole (wellhead) and the probe and the position information is combined with the azimuth and inclination information to provide a plot or map of the borehole relative to a desired coordinate system (e.g., a Cartesian coordinate system centered at the wellhead with the Z-axis extending downwardly toward the center of the earth and the X and Y axes extending in the direction of true north and true east, respectively). This early type of prior art system is subject to several disadvantages and drawbacks, including inaccuracies of the devices utilized to indicate azimuth and inclination and, of particular relevance to the present invention, inability to precisely determine the length of the cable that supports the probe or tool.
Various considerations have brought about an ever increasing need for borehole surveying apparatus that is more precise and compact than the above discussed type of prior art arrangements. For example, modern gas and oil drilling techniques often require that wells be closely spaced and, in addition, it is not unusual for a number of wells to be drilled toward different geological targets from a single wellhead or drilling platform. Further, depletion of relatively large deposits has made it necessary to drill deeper and to access smaller target formations. Even further, in the event of a deep, high-pressure blowout, precise knowledge of the borehole path is required so that a relief well can be drilled to intercept the blowout well at a deep, high-pressure formation.
One proposal for providing a small diameter probe for a borehole survey system involves the application of inertial navigation techniques that previously have been employed to navigate aircraft, spacecraft and both surface and subsurface naval vessels. Generally, speaking, these inertial navigation techniques utilize an instrumentation package that includes a set of accelerometers for supplying signals that represent acceleration of the instrumentation package along the three axes of a Cartesian coordinate system and a set of gyroscopes for supplying signals representative of the angular rate at which the instrumentation package is rotating relative to that some Cartesian coordinate system. Two basic types of systems are possible: gimballed systems and strapdown systems. In gimballed systems, the gyroscopes and accelerometers are mounted on a fully gimballed platform which is maintained in a predetermined rotational orientation by gyro-controlled servo systems. In effect, this maintains the accelerometers in fixed relationship so that the accelerometers provide signals relative to a coordinate system that is substantially fixed in inertial space, e.g., a Cartesian coordinate system wherein the Z-axis extends through the center of the earth and the X and Y axes correspond to two compass directions. Successive integration of the acceleration signals twice with respect to time thus yields signals representing the velocity and position of the instrumentation package in inertial space (and, hence, the velocity and position of the aircraft, ship or probe of a borehole survey system).
In strapdown inertial navigation systems, the gyros and accelerometers are fixed to and rotate with the instrumentation package and hence with the aircraft, naval vessel or borehole survey probe. In such a system, the accelerometers provide signals representative of the instrument package acceleration along a Cartesian coordinate system that is fixed relative to the instrumentation package and the gyro outputs are processed to transform the measured accelerations into a coordinate system that is fixed relative to the earth. Once transformed into the desired coordinate system, the acceleration signals are integrated in the same manner as in a gimballed navigation system to provide velocity and position information.
Regardless of whether a borehole survey system is implemented with gimballed or strapdown techniques (or a hybrid configuration wherein the accelerometers are gimballed relative to one or more axes of rotation), currently available accelerometers and gyroscopes do not provide satisfactory positional accuracy, unless the system is compensated or "aided." For example, the positional accuracy of a borehole survey system utilizing currently available accelerometers and gyroscopes having an accuracy of one nautical mile per hour will drift between 1,500 and 3,000 feet during a 30-minute survey. Such an error is approximately two orders of magnitude greater than that necessary to precisely survey relatively deep boreholes.
Conceptually speaking, aiding an inertial navigation system to improve long-term stability involves comparing the position or velocity signals provided by the inertial navigation system with position or velocity or velocity signals that are obtained from another source to thereby provide error signals. Since the dynamics associated with the propagation of errors within an inertial navigation system are relatively well known, the error signals can be processed to continuously or periodically modify the signal processing performed by the navigation system. One technique that has been proposed for aiding borehole navigation systems is to periodically stop the probe. The velocities indicated by the system with the probe at rest are error signals that can be utilized to estimate the true state of the system and various error parameters associated with the inertial instruments.
Repeatedly stopping the probe during a survey is undesirable in that it substantially increases the time required for the survey operation and thus results in higher costs. Moreover, to provide a high degree of accuracy, the probe must be stopped frequently or, in the alternative, the data collected during periods of time in which the probe is moving must be analyzed after the survey is complete to at least partially eliminate navigation errors that occur between the periods of time in which the probe is brought to rest.
One technique for minimizing or eliminating the need to stop the probe involves comparing an inertially derived estimate of the borehole path length between the probe and borehole entrance opening with a path length signal that is based on measuring the cable fed into or withdrawn from the borehole representative of the length of the cable that supports the probe. Specifically, in a strapdown borehole navigation system in which the Z-axis of the probe reference coordinate system extends along the longitudinal centerline of the probe, integration of the Z-axis accelerometer signal twice with respect to time provides a calculated position signal that is theoretically equal to the distance that the probe has traveled along the borehole and, hence, ideally is equal to the distance between the wellhead and the probe (as measured along the path of the borehole). If the actual length of the cable that supports the probe (i.e., extends between the wellhead and the probe) were known, it then would be possible to combine the inertially derived position signal with a signal representing the actual cable length to obtain an error or difference signal that can be utilized for precise aiding of the inertial navigation system.
The simplest approach to obtaining such a cable length signal is to measure the cable as it passes into or out of the borehole. At least two primary problems are encountered in applying this technique. Firstly, a signal must be generated that continuously and accurately represents the length of cable that is payed out or reeled in. Secondly, the technique must account for changes in cable that result because of stretching of the cable, including changes in cable stretching that occur when the probe cannot move at the rate at which cable is payed out or reeled in (because frictional forces stop or slow the probe, or because of an excessive cable feed rate).
Various prior art proposals have greatly reduced the problem of accurately determining the length of cable that is payed out or reeled in. For example, relatively accurate results are obtained by systems wherein the cable is directed through a pulley of predetermined radius at or near the point at which the cable passes into the wellhead. In most such systems, an associated electronic circuit provides a pulse signal each time the pulley rotates through a predetermined arc. The number of signal pulses are counted to provide an indication of the amount of cable that has passed into or out of the borehole. In some such prior art systems, compensation is provided for environmental factors such as frozen mud or other foreign material that, in effect, changes the radius of the measurement pulley.
Prior art proposals for compensating the measured cable length for cable stretch have been less satisfactory than systems for indicating the length of cable that passes into and out of the wellhead. For example, in the cable stretch compensation technique disclosed in U.S. Pat. No. 3,490,150, a first force measurement is made at the wellhead (e.g., as the cable passes through the measuring pulley) and a second force measurement is made at the probe. The measured forces are then combined with an estimate of the elastic compliance of the cable to provide an estimate of the amount by which the cable is stretched. One drawback of such a system is that very accurate force measurement devices are required which cannot easily be incorporated in the system probe or in wellhead equipment. Another drawback is that such a system exhibits relatively poor dynamic accuracy. In this regard, cable strain and, hence, the cable stretch is a function of both the force exerted on the cable and the temperature of the surrounding environment. Since borehole temperature increases with borehole depth, a cable moving through the borehole is exposed to temperature gradients that affect cable length. Further, the mechanical strain exerted on each incremental section of the cable is a function of: (a) the weight of the probe and weight of the cable located below that incremental section (which are functions of the borehole path [inclination] as well as probe and cable mass); (b) the frictional forces that are exerted on the probe by the surrounding walls of the borehole; and, (c) the frictional forces that are exerted on each incremental section of the cable that is within the borehole. Since each of these parameters can vary along the course of a borehole, simply measuring the force exerted on the cable at the wellhead and at the probe cannot provide totally satisfactory compensation for cable stretch.
An arrangement that partially overcomes these prior problems of using cable measurement to determine probe position is disclosed in U.S. Pat. No. 4,545,242. In that arrangement, a Kalman Filter is utilized to estimate probe length and probe velocity based on cable measurement and probe acceleration. If the probe becomes stuck in the borehole the estimate of probe depth is maintained constant by altering the values of various parameters utilized in the Kalman filtering process. When the probe resumes movement, the altered parameters gradually are returned to normal as a function of the time duration during which the probe was stuck. Although this proposal appears to be an improvement over previous attempts to continuously and accurately measure probe depth, the arrangement is somewhat complex. Further, the arrangement may not provide accurate results during time intervals during which the probe resumes movement after being stuck or during time intervals during which the probe does not stick tightly, but is slowed because of, for example, a constriction within the borehole.